1. Field of the Invention
The present invention relates to a torque detecting apparatus which detects an amount of torsion of a shaft using a sensor and detects the torque applied upon the shaft.
2. Description of Related Art
An ordinary automobile comprises a power steering apparatus to make it possible for a driver to reduce the operation power applied upon a steering wheel and make it easy for the driver to drive his automobile while he operates the steering wheel, that is, during steering, and in recent years, an ordinary automobile often comprises an electricity-powered steering apparatus (electric power steering apparatus). An electric power steering apparatus is structured such that as the torque generating in response to rotations of a steering wheel is detected, a driver's steering intention such as the steering direction and the steering amount is determined, and based on this result, a steering assisting electric motor is turned ON/OFF and the rotation force while this motor is ON is adjusted.
By the way, the torque which generates as a driver rotates a steering wheel is detected usually as the amount of torsion of a torsion bar which connects two halves of a steering shaft along the longitudinal direction of the steering shaft which rotates as the steering wheel rotates. Various types of sensors are usable to detect the amount of torsion of such a torsion bar and a Hall-effect sensor is one of them.
A magnet is fixed to an object whose amount of torsion is to be detected, namely, a movable object and a Hall-effect sensor is usually disposed at a fixed position. From a voltage outputted from the Hall-effect sensor in accordance with the positions of the Hall-effect sensor and the magnet relative to each other when a reference voltage is applied upon the Hall-effect sensor, the relationship between the positions of the Hall-effect sensor and the magnet is detected.
Since a characteristic of a Hall-effect sensor is that its gain, midpoint or the like varies depending upon a change of a power source voltage, even when the result of detection is the same, a change of the power source voltage changes an output voltage. Due to this, it has been difficult to use a Hall-effect sensor as a torque sensor for an electric power steering apparatus.
A conventional electric power steering apparatus is equipped with the so-called failsafe function of detecting with two sensors whose characteristics are identical substantially at the same time for detection of the amount of torsion of a torsion bar, determining that some abnormality has occurred when the two sensors have detected differently and prohibiting steering assisting by the power steering apparatus.
As such a failsafe function of an electric power steering apparatus which uses torque sensors to handle abnormality, there are known techniques such as those described in Japanese Patent Publication No. 4-53748 (1992), Japanese Patent No. 3094483 and Japanese Patent Publication No. 8-18564 (1996). These will now be described specifically.
Assuming that the actual value of the power source voltage Vcc which is supplied in common to the two sensors, the output voltage from the first sensor is VT1 and that from the second is VT2. In this case, the output voltages VT1 and VT2 from the two sensors are expressed by equations (1) and (2) below:VT1=(a×θ+(Vcc/2))×(Vcc/Vref)  (1)VT2=(−a×θ+(Vcc/2))×(Vcc/Vref)  (2)
Where, θ: amount of torsion of the torsion bar                a: constant (gain)        Vref: reference voltage (e.g., 5 V) for the sensors        Vcc/Vref: ratiometric constant of the sensors VT1=−VT2 should hold from the equations (1) and (2) in principle and particularly when the torsional angle θ of the torsion bar is zero, VT1=VT2 and it then follows that the output characteristics of the two sensors are cross-characteristics.        
In light of this, sensor clump voltages common to the two sensors are determined as follows.
VclumpH: sensor output upper limit voltage
VclumpL: sensor output lower limit voltage
Hence, it is judged there is something wrong with the first sensor when VT1<<VclumpL or VT1>>VclumpH, whereas when VT2<<VclumpL or VT2>>VclumpH, it is judged there is something wrong with the second sensor.
It is also possible to judge in the manner below whether the voltages fed to the sensors from the power source are abnormal. That is, an equation (3) holds true from the above equations (1) and (2).VT1+VT2=(Vcc2/Vref)  (3)
Assuming that Vcc, namely the power source voltage is a constant value, since Vref is a constant, “VT1+VT2” should also be a constant value. However, “VT1+VT2” may not yield a constant value, as the detection accuracies of individual sensors are slightly different from each other. Hence, an upper limit value (Vmax) and a lower limit value (Vmin) which are obtained by slightly widening “Vcc2/Vref” are set as threshold values, and it is decided that (the absolute values of) the output voltages from the two sensors are about the same as long as “VT1+VT2” stays within a range which satisfies an equation below. In short, when the equation below is not met, it is judged that there is a major error in the output from any one of the sensors (and in some cases, in the outputs from the both sensors):Vmin<VT1+VT2<Vmax
Where it is assumed that Vcc=Vref, i.e., when the actual power source voltage is equal to the reference voltage for the sensors, VT1+VT2=Vcc. In other words, the total of the outputs from the both sensors becomes equal to the power source voltage. Hence, monitoring of the value (voltage value) VT1+VT2 realizes whether the power source voltage is abnormal.
Each of the structures according to the conventional techniques mentioned earlier is that as for the sensor outputs, one of the two sensors is used as a main sensor, the above mentioned abnormality detection is executed, and when it is judged that there is nothing abnormal about the output voltages from the both sensors or the power source voltage, the output voltage from the main sensor is used as the sensor output. Hence, the power source voltage could vary to such an extent which can not be regarded abnormal, which may be inevitable in the case of abnormality occurring in the sensors themselves. However, according to the conventional techniques mentioned earlier, even a change of the power source voltage to such an extent which can not be regarded abnormal will be judged as an abnormal power source voltage and will operate the failsafe function.